Help balancing Xanathar fight

[Provo]

The party I DM for has finished dragon heist and is now facing down the Xanathar in Undermountain. Next session begins with rolling initiatives to fight him. I’m looking for tips to make this fight difficult enough. To note, average party level is 7 or 8, and the Wizard is 9

They beat him once already. They were in his anti-magic cone 80 ft beneath him, and they were low resources. However they took him out in just over a turn. He only got to shoot one eye beam due to his cone.

Now, I’m concerned that he is even weaker in his base. They are encountering him in a hallway (just 20 ft high), and they have full resources. If the Xanathar holds them in his cone, he can’t harm them. If he doesn’t hold them in his cone, the Wizard will use hold monster.

The Xanathar does not have powerful allies to call on, only beholderkin and unintelligent henchmen (game went off the rails. Players made the Xanathar so paranoid that he killed off all his lieutenants, and charmed everybody else until their brains were mush. He destroyed the anti-dream statue, and his lair is filling with beholderkin)

Any ideas? Is there anyway I can run a hallway fight with this guy without him just getting stomped all over? He DOES have a ring of invisibility, but he won’t run because the players have his fish.

[ProsecutorGodot]

Well, It's in the lair so that should make things a bit easier. The Xanathar is not stupid, just paranoid. It can use his lair actions to grapple or hinder the parties movement as well as shoot an eye laser from a different position. You can choose the direction of the cone at the start of its turns so it isn't forced to use it in a way that would render himself unable to fight back. Here are a few things of note:
-The lair action that allows beholders to spawn grappling appendages on walls will be particularly potent in such a confined area.
-Its Eyebeam range is 120ft, which far outranges Hold Monster.
-Xanathar has a reasonable chance of saving from Hold Monster, which I assume is around DC 16-17
-There's no reason for it to fight fair and start in a position where it can be attacked. Why not start the fight invisible?
-If you're really concerned that the Xanathar won't stand up to snuff on its own, this is a great time for a rival faction to intercede as a third party.
-Legendary actions, constantly. The threat of a random eye beam at the end of each players turn should help.

I don't really have hands-on experience with how powerful The Xanathar can be. My party avoided him like the plague during Dragonheist and has only recently gotten to the point where they might be comfortable with trying to kill him. Just play him ruthlessly and make ample use of the resources he's allowed (Lair Action, Legendary Action, Invisibility) and you should be able to at least make he fight more memorable than that time the party gunned down a beholder in some narrow corridor.

[Vogie]

Add some Gazers or similar beholderkin creatures that flank the party
Have a group of thuggish types drop in (literally rappel down) behind the party after they engage Xanathar
He has magic mirrors that allow him to see the party and start hitting them with eye beams before they can see him
He does things with the floor that wouldn't impact him as much because he is floating, such as traps, glyphs of warding, Snares, acid pools or lava flows
He could have a dark priest(s) use Hallow spells to grant zones of darkness, silence, antiportal zones, or Energy Vulnerability.
He could engage them in a guerilla fight beforehand, attacking from strange angles, then scooting around invisibly, then attacking again.

Remember, he's already faced them once, and they had overcome his normal defenses. He also knows how they fought last time, and would have put things in place to stop how they defeated him last time. For example, if they hung out in the antimagic zone and used powerful weapons, he'd be trying to disarm them, and have some sort of trap, like acid or lava, open up under them if they try the same tactics again.

[bc56]

The big thing to note is that beholders don't fight fair at all, Xanathar especially. Reasonably, he wouldn't fight in a hallway where his two biggest advantages are negated.
And those advantages are pretty simple. Beholders have far longer darkvision ranges than most PCs, allowing them to snipe with eye rays from out of sight, and they can fly out of the range of many attacks. The comments about legendary and lair actions are also appropriate

[Psikerlord]

To make a true boss solo monster for 5e, I recommend:

1. Double hp.
2. Boss gets Off Turn Attacks (it gets an attack in between every PC turn - this largely balances the action economy, at least up to about 10th)
3. Boss gets to make a save against things that dont normally allow a save (like sleep or forcecage).
4. Boss gets a small Reroll Pool (eg 1d4) that it can use for anything.

That's pretty much it. Throw this template on your beholder and have at it.

[Ganymede]

I take cues from lots of video game media and add in hard HP thresholds and phase changes. It worked great when I curated some boss monsters for my Curse of Strahd campaign and could work here.

[Provo]

Thank you for the tips everybody. The fight happened today and went great.

As suggested, the Xanathar used mirrors to shoot around corners (which the players loved), this kept him out of range and line of sight while weak henchmen blocked the way. Of course, legendary actions, lair actions, and environmental dangers were in play. He did not use his anti-magic cone as he knew it would hinder him in this case.

Due to poor RNG early on, all but one player died. However, they had built up resources to bring back characters. Everybody seemed very satisfied. Thanks again for helping with a memorable fight.

[Malifice]

Give it Legendary resistances. 3/ Long rest.

Done. It is a named NPC in its lair for Torms sake.

Personally I'd also add +2 to all saves, attack rolls and ability DC's, increase HP by 50 percent, and increase all damage dice by +1 step (This is the Xanathar, not some ordinary beholder). I'd also allow it to choose which ray to blast each turn.

As for tactics, place the Wizard in the AM cone, and then target the with the [B]dominate ray (and order him to attack the Wizard or other squishy target), target the [Rogue/ glass cannon with a poor Con/ Str save] with the Enervation ray/ TK ray, and target the [Cleric with the poor Dex save in heavy armor] with the Disintegrate ray.

Do this while you take advantage of flight. Using antimagic, no creatures without natural flight can reach you to melee.

He's smart. Play him smart.

Also; he's probably paid someone for a Clone. He's pretty paranoid like that.

[Provo]

Give it Legendary resistances. 3/ Long rest.

Done. It is a named NPC in its lair for Torms sake.

Personally I'd also add +2 to all saves, attack rolls and ability DC's, increase HP by 50 percent, and increase all damage dice by +1 step (This is the Xanathar, not some ordinary beholder). I'd also allow it to choose which ray to blast each turn.

As for tactics, place the Wizard in the AM cone, and then target the with the [B]dominate ray (and order him to attack the Wizard or other squishy target), target the [Rogue/ glass cannon with a poor Con/ Str save] with the Enervation ray/ TK ray, and target the [Cleric with the poor Dex save in heavy armor] with the Disintegrate ray.

Do this while you take advantage of flight. Using antimagic, no creatures without natural flight can reach you to melee.

He's smart. Play him smart.

Also; he's probably paid someone for a Clone. He's pretty paranoid like that.

Yeah, the fact that he uses a regular beholder stat block is a bit ridiculous, even with his magic items.

Ultimately, I played him as smart, but not as smart as a beholder in its lair would normally be. Not only was he missing important assets due to killing off his own lieutenants, but the players were trying to steal Sylgar. Xanathar was desperate to stop them, so he couldn’t stage the fight on his terms.

[Malifice]

Yeah, the fact that he uses a regular beholder stat block is a bit ridiculous, even with his magic items.

Ultimately, I played him as smart, but not as smart as a beholder in its lair would normally be. Not only was he missing important assets due to killing off his own lieutenants, but the players were trying to steal Sylgar. Xanathar was desperate to stop them, so he couldn’t stage the fight on his terms.

Nice.

I would definately have upgunned him a fair bit (my general rule for an 'advanced' monster is +2 to AC, Saves, DCs, Attack rolls; and increase damage dice on all attacks by +1 step per dice, +50 percent HP, +2 CR).

As a player I'd capture the Fish, and store it in a magical bowl of force (if the magic is knocked out, the bowl vanishes and the Fish bites the dust).

'One false move Xanathar, and the Fish gets it!'.